Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
The result of this is the emergence of micro-electrical-mechanical-systems (MEMS) based transducer devices. These may be for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. There is a continual drive to reduce the size and cost of these devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of transducers, the device is driven by a potential difference provided across the electrodes.
FIGS. 1 and 2 show a schematic diagram and a perspective view, respectively, of a known capacitive microphone device. The capacitive microphone device comprises a flexible membrane 1 that is free to move in response to pressure differences generated by sound waves. A first electrode 3 is mechanically coupled to the flexible membrane 1, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 5 is mechanically coupled to a generally rigid structural layer or back-plate 7, which together form a second capacitive plate of the capacitive microphone device.
The capacitive microphone is formed on a substrate 9, for example a silicon wafer. A back-volume 11 is provided below the membrane 1, and is formed using a “back-etch” through the substrate 9. A plurality of openings 13, referred to hereinafter as acoustic holes, are provided in the back-plate 7 so as to allow free movement of air molecules, such that the sound waves can enter a cavity 15 above the membrane 1. A plurality of openings 17, hereinafter referred to as bleed holes, connect the cavity 15 with the back-volume 11. The cavity 15 and back-volume 11 allow the membrane 1 to move in response to the sound waves entering via the acoustic holes 13 in the back-plate 7.
Thus, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane 1 is deformed slightly from its equilibrium position. The distance between the lower electrode 3 and the upper electrode 5 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown).
FIG. 2 shows a perspective view of the MEMS device illustrated in FIG. 1.
FIG. 3 shows a simplified cross-sectional view of a conventional MEMS device such as that shown in FIGS. 1 and 2. As mentioned above, a first electrode (not shown) forms part of the flexible membrane 1, and a second electrode (not shown) is attached to or embedded in the rigid back-plate 7 above the membrane 1. Both the back-plate 7 and the membrane 1 may be formed from silicon nitride, for example, and the substrate from silicon. However, the thermal expansion coefficient of silicon is greater than that of silicon nitride and this leads to stresses at the interface between the two dissimilar materials.
The structure of FIG. 3 is formed by various processes of depositing layers and then selectively dry or wet etching portions of the layers away again. These processes take place at relatively low temperatures (in the order of 10-400° C.). When the layers are deposited, there are no intrinsic stress concentrations in the structure. When the structure is released by removal of the sacrificial layers the tensile stress of the deposited layer causes a torsional moment in the membrane sidewall. This leads to a tensile stress concentration on the outer sidewall edge and a compressive stress concentration on the inner sidewall edge. A similar stress can be found in the membrane 1.
These stress concentrations tend to cause cracking originating at the points labelled A and B in FIG. 3, and can lead to failure of the MEMS device. This stress can also render the MEMS device more susceptible to failure during fabrication. For example, when multiple MEMS devices are fabricated on a single wafer and subsequently separated using a technique known as singulation or dicing, the stress at points A and B can cause the device to crack and fail. After failure at these points, the transducer is rendered useless.
It is therefore an aim of the present invention to provide a MEMS device that does not suffer from the disadvantages mentioned above.